Historically, the role of NO in human health was thought to be primarily as an irritant in air pollution. This view changed dramatically in 1987 with the discovery of the production of NO in the body and the identification of the molecule's key role in biological signaling. Since 1987, thousands of scientific papers on NO have been published, which attests to the intensity of research interest devoted to this molecule. In recognition of the medical significance of the molecule, the 1998 Nobel Prize in Physiology was awarded to the scientists who discovered the role of NO as a biological messenger.
NO is a molecular messenger synthesized by nitric oxide synthase (NOS) from L-arginine and oxygen. NO is involved in a number of physiological and pathological processes in mammalians. Three structurally distinct isoforms of NOS have been identified: neuro (nNOS), endothelial (eNOS), and inducible (iNOS).
NO is a small gaseous molecule with chemical properties that make it uniquely suitable as both an intra- and intercellular messenger. Because it possesses an unpaired election, NO reacts with other molecules with unpaired electrons, especially superoxide, which can combine with NO to form peroxynitrite, a highly reactive and toxic radical. As a neutral gaseous molecule, NO can diffuse over several cell lengths from its source to exert control over certain enzymes and regulate key cellular functions. The combined properties of its ability to regulate enzymes across long distances as well as its high reactivity with other molecules give NO its unique dual role as both a powerful signaling molecule and a lethal effector molecule.
Because of these powerful functions, the production of NO is tightly regulated and there is ample literature to show that too little or too much NO production contributes to numerous human diseases and disorders.
NO is a potent pleiotropic mediator of physiological processes such as smooth muscle relaxation, neuronal signaling, inhibition of platelet aggregation and regulation of cell mediated toxicity. It is a diffusible free radical which plays many roles as an effector molecule in diverse biological systems including neuronal messenger, vasodilation and antimicrobial and antitumor activities. NO appears to have both neurotoxic and neuroprotective effects and may have a role in the pathogenesis of stroke and other neurodegenerative diseases, in demyelinating conditions (e.g., multiple sclerosis), in ischemia and traumatic injuries associated with infiltrating macrophages, and in the production of proinflamatory cytokines. A number of pro-inflammatory cytokines and endotoxin (bacterial lipopolysaccharide, LPS) also induce the expression of inducible nitric oxide synthase (iNOS) in a number of cells, including macrophages, vascular smooth muscle cells, epithelial cells, fibroblasts, glial cells, cardiac myocytes as well as vascular and non-vascular smooth muscle cells.
Although NO mediates a number of physiological functions, overproduction of NO has been reported in a number of clinical disorders. Therefore, maintaining suitable levels of NO is very important. For example, decreased NO generation in the penis results in impotence. On the other hand, many other diseases and conditions such as intradialytic hypotension, hemorrhagic shock, tissue rejection, rheumatoid arthritis, and diabetes are associated with the overproduction of NO.
There is now substantial evidence that excess NO production is involved in a number of conditions, including conditions that involve systemic hypotension such as septic and toxic shock and therapy with certain cytokines. Circulatory shock of various etiologies is associated with profound changes in the body's NO homeostasis. In animal models of endotoxic shock, endotoxin produces an acute release of NO from the constitutive isoform of nitric oxide synthase in the early phase, which is followed by induction of iNOS. NO derived from macrophages, microglia and astrocytes has been implicated in the damage of myelin producing oligodendrocytes in demyelinating disorders like multiple sclerosis and neuronal death during neuronal degenerating conditions including brain trauma.
Cytokines associated with extracellular signaling are involved in the normal process of host defense against infections and injury, in mechanisms of autoimmunity, and in the pathogenesis of chronic inflammatory diseases. It is known that NO mediates deleterious effects of the cytokines. For example, NO as a result of stimuli by cytokines (e.g., TNF-α, IL-1, and/or IL-6) is implicated in autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, osteoarthritis, inflammatory bowel disease, systemic lupus erythematosus, and diabetes. The NO produced by iNOS is associated with bactericidal properties of macrophages. Recently, an increasing number of cells (including muscle cells, macrophages, keratinocytes, hepatocytes and brain cells) have been shown to induce iNOS in response to a series of proinflammnatory cytokines including IL-1, TNF-α, interferon-γ (IFN-γ) and bacterial lipopolysaccharides (LPS).
There are several drugs being used to treat diseases associated with the overproduction of NO and/or cytokines. The present invention relates to the significant improvement on a marketed drug, leflunomide (marketed as Arava® by Aventis Pharmaceuticals).
Leflunomide has been approved for treating rheumatoid arthritis (RA) by the Food and Drug Administration (FDA) since 1998. Recently, it was approved for an additional indication, improvement in physical function. Currently, leflunomide is being developed for multiple sclerosis.
Highly reactive NO, generated by astrocytes and infiltrating macrophages, is implicated in inflammatory destruction of brain tissue, including that occurring in multiple sclerosis. Leflunomide was shown to inhibit activation of iNOS in rat astrocytes. (Miljkovic D. et al., “Leflunomide inhibits activation of inducible nitric oxide synthase in rat astrocytes”. Brain Res. Jan. 19, 2001; 889(1-2): 331-8.)
